1. Field
This invention pertains to optical measurement instruments, and in particular a method of verifying the correct operation of an optical measurement instrument by using a model eye, and an optical measurement instrument that employs such a method.
2. Description
It is sometimes necessary to be able to verify correct operation and specified performance of an optical measurement instrument such as a wavefront aberrometer in an operational setting. In many instances, this is done by operating the optical measurement instrument to make a measurement of a model eye whose characteristics are known. A common version of a model eye is a solid glass or plastic component with a curved front surface and a flat back surface. The front curve serves the role of a “cornea” for the model eye, and the back surface serves as a “retina” for the model eye. Some model eyes have a limiting aperture that serves as an “iris” for the model eye. The aperture is most commonly located in front of the front surface of the model eye, but it can also be inside the model eye.
To verify correct operation and specified performance of an optical measurement instrument, typically the optical measurement instrument injects a probe beam into a front surface of the model eye. Light scatters from the back surface of the model eye similarly to the way it does with a human eye, and some of the scattered light travels back out of the front surface of the model eye and into the optical measurement instrument. From the received light, the optical measurement instrument makes one or more measurements of the model eye. Typically, the optical measurement instrument measures the sphere and/or cylinder values of the model eye, and compares these measured values with corresponding predetermined calibration data for the model eye to determine whether or not the optical measurement instrument is operating properly. The values must agree within some tolerance for the optical measurement instrument to be considered in good working order.
However, the values for sphere and cylinder that an optical measuring instrument measures will vary depending on the angle that an axis normal to the model eye makes with respect to the optical measurement instrument's optical axis (hereinafter referred to as “the misalignment angle”). The predetermined calibration data assumes a misalignment value of zero degrees. Even when the optical measurement instrument is operating perfectly, when the misalignment angle is not zero degrees then there will be a variance between the measured sphere and cylinder values for the model eye and the predetermined calibration values. This variation in the measured sphere and/or cylinder values that depends on the misalignment angle between the optical measurement instrument and the model eye makes it hard to verify proper operation of the optical measurement instrument. For example, experiments have been performed with an example optical measurement instrument making measurements on a model eye that has a front surface curvature that matches that of a human cornea. With a misalignment angle of only three (3) degrees, the model eye measurements were 0.5 Diopters different from the calibration value for perfect alignment (i.e., zero degree misalignment angle). This variation was far in excess of the maximum tolerable variation of 0.1 Diopters for the example optical measurement instrument.
It should be noted that the problem described here is unique to measuring model eyes. This misalignment does not occur when measuring a human eye, because the patient directs their gaze straight into the optical measurement instrument to view a fixation target of the optical measurement instrument, thus automatically aligning the human eye with the optical axis of the optical measurement instrument.
In contrast to this simple method of aligning a human eye by means of a fixation target, it may be difficult and/or time-consuming for an operator through trial-and-error to achieve a degree of alignment between a model eye and the optical measurement instrument's optical axis that renders insignificant the variation in the measured sphere and/or cylinder. One solution is to constrain the model eye by mechanical means so it points directly toward the optical measurement instrument. However, this approach adds expense to the model eye mount and may not reduce the measurement variation to within a desired tolerance.
Therefore, it would be desirable to provide a method of verifying proper operation with an optical measurement instrument with a model eye that can address variations in measurements that occur when the model eye is misaligned with respect to the optical measurement instrument. It would also be desirable to provide an optical measurement instrument that can operate with such a method.
In one aspect of the invention, a method comprises: receiving a light beam from a model eye at an optical measurement instrument having an optical axis; producing image data, including light spot data for a plurality of light spots, from the received light beam; determining an observed location of a corneal reflex from the model eye within an image representing the image data; and determining an angle of misalignment between an axis normal to the front surface of the model eye and the optical axis of the optical measurement instrument from the observed location of the corneal reflex within the image.
In another aspect of the invention, a measurement instrument comprises: one or more light sources configured to illuminate a model eye; a light spot generator configured to receive light from the model eye and to generate a plurality of light spots from the light received from the illuminated object; a detector configured to detect the light spots and for outputting image data, including light spot data for the plurality of light spots; and a processor. The processor is configured to process the image data to determine an alignment between the measurement instrument and the model eye by: determining an observed location of a corneal reflex from the model eye within an image representing the light spot data; and determining an angle of misalignment between an axis normal to the front surface of the model eye and an optical axis of the measurement instrument from the observed location of the corneal reflex within the image.
In yet another aspect of the invention, a method is provided for determining a misalignment between a measurement instrument and a model eye used to verify correct operation of the measurement instrument, by determining a difference between: (1) an observed location of a corneal reflex in an image produced by the measurement instrument from the model eye, and (2) an expected location of the corneal reflex.
In still another aspect of the invention, a method comprises: receiving a light beam from a model eye at an optical measurement instrument having an optical axis; producing image data, including light spot data for a plurality of light spots, from the received light beam; determining an observed location of a corneal reflex from the model eye within an image representing the image data; defining an analysis area within an image represented by the image data, wherein the analysis area is centered on the observed location of the corneal reflex; and measuring at least one of a sphere value and a cylinder value for the model eye from a portion of the light spot data corresponding to light spots within the analysis area.